Hypothesis: The negative pressure in liquids under the concave meniscus of nanometer size can be observed experimentally. This allows verification of the predictions of the macroscopic Young-Laplace law, which has so far been performed only on the basis of theoretical calculations. The deviation of the negative pressure from the Young-Laplace law allows to get information about the structure of the porous matrix. Experiments: The properties of n-heptane confined in nanometer-sized spaces are monitored in situ due to the particular ability of positronium to form subnanometer bubbles in liquids. Positron annihilation lifetime spectroscopy allows to detect their sizes, which in turn are used to determine surface tension and negative pressure of the investigated liquid. These results are obtained by means of an improved quantum-mechanical model approximating the o-Ps bubble with a 1.3 eV finite potential well. Findings: The dependence of the negative pressure on the curvature of the concave meniscus is found, which follows the Young-Laplace law with a good accuracy for model cylindrical pores. The differences between the pressure in the liquid confined in pores of various sizes and shapes in different materials allow inference about several phenomena (e.g. pore blocking in materials with a complex pore structure or swelling of the polymer). (C) 2019 Elsevier Inc. All rights reserved.